The method according to the present invention can be applied after carrying out a process wherein acrylic acid-containing gas obtained from the vapor phase oxidation of propylene and/or acrolein in the presence of a catalyst is contacted with an absorbing solvent to collect acrylic acid in a solution state, the solution is rectified to recover crude acrylic acid, and the crude acrylic acid is purified in a separation tower to remove impurities with high boiling point.
In general, because the crude acrylic acid obtained by the above-described oxidation method contains impurities with high boiling point such as acrylic acid dimer and maleic acid, it is not economically advisable to discard such impurities as waste liquid due to the loss of acrylic acid. Acrylic acid has high polymerizability and polymerization of acrylic acid may occur during the above process. However, in addition to such polymerization, oligomerization such as dimerization and trimerization of acrylic acid may occur with ease. It is thought that such oligomerization results from serial reactions. During the above purifying process based on distillation, acrylic acid dimer is produced chiefly.
Production of acrylic acid dimer depends on temperatures and retention time. Additionally, it is not possible to inhibit production of acrylic acid dimer completely by adding a polymerization inhibitor. In conventional distillation processes, it is not evitable that acrylic acid dimer is produced in an amount of 1-5 wt %. Even if the impurities with high boiling point is removed by distillation, 5-50 wt % of acrylic acid dimer may be concentrated at the bottom of the distillation column. Therefore, it is not cost-efficient to discard the bottom liquid containing a high concentration of acrylic acid dimer without any post-treatment.
To solve the above problems, methods for recovering acrylic acid by pyrolysis of acrylic acid dimer into acrylic acid are suggested in Japanese Patent Publication Nos. 1970-19281B2, 1976-91208A, 1986-36501B2, 1999-12222A, etc.
The method disclosed in Japanese Patent Publication No. 1999-12222A uses an acrylic acid recovering column having a distillation tower and thin film evaporator, separated from a dimer pyrolysis unit. However, the method has a problem in that polymerization of acrylic acid may occur at a duct connecting the recovering column with the pyrolysis unit. Additionally, it is not cost-efficient that at least two equipments should be added for the acrylic acid recovering column and the pyrolysis unit.
Further, according to the above method, the thin film evaporator is operated at reduced pressure and the pyrolysis unit is operated at atmospheric pressure. In this regard, the method uses a condenser in order to remove odors emitted from vapors generated by acrylic acid, acrylic acid dimer and various kinds of impurities with high boiling point. Therefore, when the system is operated for a long time, polymerization of acrylic acid may occur at the part of the condenser to cause troubles in operation, resulting in emission of bad odors toward the surroundings.